L12 - Biophysics and Tissue Fluid

Question 1

What is the equation to calculate blood flow?

Answer 1

Fluid flow through vessels is governed by physical laws 
Blood flow (cm3/s) = pressure/resistance

Question 2

What is Poiseuilles Law?

Answer 2

Fluid flow = πPr4/8nL
r = radius of the tubing 
P = pressure gradient across the tubing  
n = viscosity of the fluid
L = vessel length

Question 3

What is the equation to calculate resistance?

Answer 3

Resistance = 8nL/πr4
Flow and resistance depend on P, L and n
r is critically important

Question 4

Which vessels are resistance vessels?

Answer 4

Small arteries and arterioles are where blood first meets high resistance
Hypertension drugs target resistance vessels
Vessel diameter is a key control point

Question 5

What is the difference between rigid tubes and biological vessels?

Answer 5

Rigid tubes – linear pressure-flow graph

Biological vessels – curved pressure-flow graph

Question 6

What is critical closing pressure?

Answer 6

Closed vessels – when tissue pressure exceeds outer blood pressure

Question 7

What is the Law of Laplace?

Answer 7

Transmural pressure = tension/radius

The smaller the radius the greater the pressure

Question 8

What is the equation to calculate tension?

Answer 8

Tension = Pr/W
r = internal radius
W = wall width
This value is different with aneurysms

Question 9

What is compliance?

Answer 9

Vessels walls have varying degrees of give

Question 10

What is the equation to calculate compliance?

Answer 10

Compliance = V/P
V = volume change 
P = pressure change

Question 11

What are capacitance vessels?

Answer 11

Veins are more compliant than arteries and store more blood – capacitance vessels
- Post mortem blood pooling – pools in venous system near the ground

Question 12

What is the Windkessels effect?

Answer 12

Arteries and veins are elastic 
- Vessels will expand in systole 
- Elastic recoil occurs in diastole 
- Allows continuous flow 
Pulsatile can be discontinuous and continuous

Question 13

Where does turbulent flow normally occur?

Answer 13

Usually occurs in blood vessels as allows better absorption of gases
However, means RBC and WBC get damaged quickly

Question 14

What is the Reynolds number equation?

Answer 14

Reynolds number = pDv/n
p = fluid density 
D = vessel diameter 
V = mean velocity
n = viscosity

Question 15

If Reynolds number < 2000 what flow is present?

Answer 15

Laminar

Question 16

If Reynolds number > 3000 what flow is present?

Answer 16

Turbulent

Question 17

What factors decrease as you reach arterioles?

Answer 17

Blood pressure and velocity

Question 18

How much blood is stored within the venous system at rest?

Answer 18

54%

Question 19

How much blood is stored within the capillaries?

Answer 19

5%

Question 20

What is the velocity through capillaries?

Answer 20

Velocity low – 0.07 cm/s

1-2 s to pass through

Question 21

What controls flow in capillaries?

Answer 21

Pre-capillary sphincters

Question 22

What are the roles of capillaries?

Answer 22

Major sites of oxygen, carbon dioxide and nutrient exchange

Question 23

What are the two different states of capillaries?

Answer 23

Active

Inactive - collapsed

Question 24

How is tissue fluid formed?

Answer 24

Net fluid transfer from capillaries to tissues forms tissue fluid

• Controlled by a balance of filtration and absorption
• Transcellular filtration (through cells) and paracellular filtration

Question 25

What 3 things does tissue fluid formation depend on?

Answer 25

P - hydrostatic pressure difference between capillary and interstitial fluid
COP - colloid osmotic pressure difference
CFC - capillary filtration coefficient

Question 26

What is COP?

Answer 26

Colloid osmotic pressure difference

Caused by impermeable proteins e.g. albumin

Question 27

What is CFC?

Answer 27

Capillary filtration coefficient

Ease of flow across vessel wall

Question 28

A balance of what causes net tissue fluid production?

Answer 28

Blood HP is normally higher than tissue fluid HP
- P is out
Plasma high and tissue fluid low
- COP=COP plasma so COP in